Conventionally, the interior and exterior surfaces of beverage cans and food can containers made of metal are coated with solvent-based paint composed mainly of thermosetting resin. This is for the purpose of keeping the flavor of contents, preventing corrosion of metal serving as the material of beverage cans and food can containers, or improving the design of the exterior surfaces of beverage cans and food can containers or protecting the printed surfaces. The solvent-based paint, however, has problems in terms of work safety and environmental impact, because heating at high temperatures is required for forming coatings and a large amount of solvent is produced during heating. For this reason, metal coatings with thermoplastic resin have been proposed as a corrosion prevention method that does not require solvent. Among thermoplastic resins, especially polyester resin is excellent in formability, heat-resistance, and the like. In this respect, films for metal lamination based on polyester resin have been under development.
When a laminated metal sheet (resin-coated metal sheet) formed by laminating (coating) a resin film such as polyester is applied to a lid member for beverage cans or a lid member for food can containers, the lid member is seamed at high speed for improving productivity to cause film cracking or film peeling on the exterior film. Another problem is that the cyclic trimer in polyester resin is deposited on the resin surface during the high-temperature sterilization process such as the retort process to impair the design, or that the resin layer itself is discolored and looks whitish during the retort process (a phenomenon called whitening). On the other hand, the resin for use on the interior surface requires corrosion resistance against the contents (contents resistance) and adhesion when the resin is in contact with the contents for a long time.
Since the required performance differs between the interior and exterior surfaces of the metal sheet as described above, films also differ between the interior and the exterior of the metal sheet. When different films are used for the interior and exterior surfaces, the physical properties of the films, especially the melting point, vary to pose operational challenges in laminating the films simultaneously on the interior and exterior surfaces. That is, in order to ensure the adhesion between the film with a high melting point and the metal sheet, it is necessary to laminate the film with a high melting point at high temperatures. However, when the film with a high melting point is laminated at high temperatures, not only the interface between the film with a low melting point and the metal sheet but also the entire layer of the film with a low melting point is melted, and the film with a low melting point may adhere (fuse) to the roll for pressure-bonding the film.
A method for improving such a problem is disclosed in Patent Literature 1, which provides a double film-laminated can lid with favorable seaming resistance, in which both surfaces of a metal sheet are coated with thermoplastic resin films, the amorphous ratio of the thermoplastic resin film layer on the exterior of the can lid is 60% or more, and the oriented crystalline layer is left in part of the thermoplastic resin film layer on the interior of the can lid.
Patent Literature 2 discloses a metal sheet coated with a film on the container exterior surface, which is polyester including 30 to 50% by mass of polyester having ethylene terephthalate as a main repeating unit and 50 to 70% by mass of polyester having butylene terephthalate as a main repeating unit. This prevents the occurrence of white spots (whitening) by setting the shortest half-crystallization time to 100 seconds or less to use heat in the retort process for crystallization and increase the crystallization rate. According to the description, this metal sheet has a polyester resin layer of a double-layer structure on the interior surface of the container, and the upper polyester resin layer is polyethylene terephthalate or copolymerized polyethylene terephthalate in which isophthalic acid is copolymerized as an acid component at a ratio equal to or less than 6 mol %. In addition, the upper polyester resin layer contains 0.1 to 5% by mass of olefin-based wax, and the lower polyester resin layer is copolymerized polyethylene terephthalate in which isophthalic acid is copolymerized as an acid component at a ratio equal to or less than 10 to 22 mol %. Similarly, Patent Literatures 3 to 6 disclose techniques for improving whitening resistance of the exterior film.
Patent Literature 7 describes a polyester composition containing 30 to 50% by mass of polyester having ethylene terephthalate as a main repeating unit and 50 to 70% by mass of polyester having butylene terephthalate as a main repeating unit. The technique for thus suppressing discoloring in the retort process is described. Patent Literature 7 also describes a technique in which the melting point of the resin is defined and the interface is melted when the resin is heat-fused. Patent Literatures 8 and 9 also describe techniques for suppressing discoloring during the retort process.
Patent Literatures 10 and 11 disclose a steel sheet laminated with different films on the can interior surface and the can exterior surface. In addition, Patent Literature 10 describes a technique for improving whitening resistance by using a polyester film with a contact angle of 70 to 120° on the can interior surface and laminating PET-PBT with a crystallization temperature of 120° C. or lower on the can exterior surface. Patent Literature 11 discloses a technique of laminating PET-PBT on the can exterior surface and copolymerized PET on the can interior surface.